$4st + 2su + 9s - 4 = -3t - 9$ Solve for $s$.
Explanation: Combine constant terms on the right. $4st + 2su + 9s - {4} = -3t - {9}$ $4st + 2su + 9s = -3t - {5}$ Notice that all the terms on the left-hand side of the equation have $s$ in them. $4{s}t + 2{s}u + 9{s} = -3t - 5$ Factor out the $s$ ${s} \cdot \left( 4t + 2u + 9 \right) = -3t - 5$ Isolate the $s$ $s \cdot \left( {4t + 2u + 9} \right) = -3t - 5$ $s = \dfrac{ -3t - 5 }{ {4t + 2u + 9} }$